Administration of the diuretic and NKCC1 chloride cotransporter antagonist bumetanide reduces the severity of autism spectrum disorders in children, and this effect is mediated by a reduction of the elevated intracellular chloride concentrations and a reinforcement of GABAergic inhibition (Lemonnier et al Transl Psychiatry. 2012;2:e202; Tyzio et al Science. 2014;343:675–679). Here, we report that this treatment also reduces the severity of symptoms in an adolescent with schizophrenia. Long-term treatment reduced hallucinations significantly, suggesting that this treatment may also be useful to treat schizophrenia. Further clinical trials and experimental studies are warranted to test this hypothesis.